In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These aromatic streams also contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes.
The aromatic streams are used as feedstocks in various subsequent petrochemical processes. In certain of these processes, such as para-xylene production, e.g., from an aromatic stream containing benzene, toluene and xylenes (BTX) or toluene disproportionation, hydrocarbon contaminants cause undesirable side reactions. Therefore the hydrocarbon contaminants must be removed before subsequent processing of the aromatic streams.
Moreover, the shift from high-pressure semiregenerative reformers to low-pressure moving bed reformers results in a substantial increase in contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from the aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams such as BTX by contacting the aromatic stream with acid-treated clay. Other materials, e.g., zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material, while zeolites used for this purpose generally are synthesized and are therefore more expensive. Both clay and zeolites have very limited lifetimes in aromatics treatment services. The length of service correlates with the level of bromine reactive impurities (“BI-reactive” impurities or contaminants) in the feedstream. BI-reactive contaminants rapidly age both clay and zeolites. Indeed, although clay is the less expensive of the two alternatives, large aromatic plants can spend a significant amount of money on clay. Furthermore, since zeolites are considerably more expensive than clay, their use in removing hydrocarbon contaminants can only be justified by dramatically improved stability in aromatics treatment so that their cycle length is practical.
U.S. Pat. Nos. 6,368,496 and 6,781,023 teach bromine reactive hydrocarbon contaminants are removed from aromatic streams by first providing an aromatic feedstream having a negligible diene level. The feedstream is contacted with an acid active zeolite catalyst composition under conditions sufficient to remove mono-olefins. The aromatic stream may be pretreated to remove dienes by contacting the stream with clay, hydrogenation or hydrotreating catalyst under conditions sufficient to substantially remove dienes but not monolefins.
Other relevant references include U.S. Pat. Nos. 6,500,996; 7,214,840; and U.S. Patent Application No. 2006/0270866.
Although zeolites have proven equal or superior to clay in many commercial applications, clay has at least one remaining advantage. The clay generally produces lower levels of toluene and benzene byproducts. These byproducts are produced in clay treaters treating aromatic feeds comprising xylenes and higher aromatics. They are believed to be produced by transalkylation reactions. The zeolite catalyst is apparently more active than clay for aromatics transalkylation at constant olefin removal levels resulting in higher levels of benzene and toluene impurities in the reactor product. There is a need for methods to improve the selectivity of zeolite catalysts.
Following standard clay start-up procedures, the zeolite catalyst is first dewatered (“dried”) using available unit feedstock at the operating temperature of the parallel reactor that is on-stream, to a predetermined level, such as to a point where the water level in the effluent is <1000 ppm. Once this point (or some other desired level) is reached, the entire unit feedstock is directed to the reactor with dewatered, fresh catalyst so that the parallel reactor is ready to be brought off line and reloaded with fresh zeolite catalyst (or clay). This results in relatively high selectivity to benzene and toluene impurities.
Recently, an improved start up procedure was disclosed in U.S. Provisional Patent Application Ser. No. 61/171,553, filed Apr. 22, 2009, wherein the zeolite catalyst is first dewatered and then fresh feedstock is flowed through the reactor at temperatures significantly below normal operating conditions, such as approximately 100° C. or less, for a predetermined period of time, such as between 0.5 to 5 days. Then the temperature of the feedstock is raised to the operating temperature.
Additionally, relevant recent disclosures include U.S. Provisional Application Ser. Nos. 61/171,549 and 61/171,559, both also filed Apr. 22, 2009. In 61/171,549, reduction of bromine index is achieved by removal of trace olefins and dienes from aromatic feedstocks using start-up conditions outside the ordinary range currently used, such as, in embodiments, the feed is heated and contacts the zeolite catalyst above temperatures currently used, such as about 210° C., and the temperature is gradually increased to between about 240 and 300° C. at the end of the cycle. In 61/171,559, a catalyst regeneration is described wherein a small amount of coke is intentionally left on the catalyst, said regeneration found to result in an improvement in the activity of the regenerated catalyst.
The present inventors have noted that when zeolite is used to treat aromatic hydrocarbon feedstreams, such as heavy reformate or isomerate, any benzene and toluene that is produced via side reactions contaminates the overhead product of the xylene fractionator, located downstream. This applies particularly to cases where the catalyst is selected from one or more zeolites selected from MCM-22, MCM-36, MCM-49, MCM-56, EMM-10 or such zeolites co-loaded with clay. This overhead stream can either be a mixed xylene product or the feed stream of the paraxylene purification unit. If the C8 aromatic product is fed to downstream processing such as a Parex™ or Eluxyl™ unit, both per se well known in the art, benzene will have an undesirable impact on the unit's adsorbent selectivity.
During the dry-out procedure when a fresh, regenerated or rejuvenataed feedstream purification unit containing zeolite (“treater”) is started-up, the system will generate a relatively large amount of benzene and toluene until the catalyst and/or clay reduces in activity. The treater must be kept off-line until the benzene and toluene co-production subsides in order to meet the paraxylene adsorber and/or mixed xylene specifications. An extended dry-out period or circulating feed through the treater while it is still off-line can result in production curtailment and can have a significant undesirable economic impact.
Without wishing to be bound by theory, it is believed that the desired reaction in the treatment process includes alkylation of olefins with contained aromatics to form heavy aromatics that can be easily removed via fractionation. Competing reactions that generate benzene are transalkylation/disproportionation of EB, xylenes and C9+ aromatics to form benzene and toluene and other reaction byproducts. The present inventors have noted that the activation energy of the alkylation reactions are thought to be lower than the other, undesirable side reactions. Accordingly, the present inventors have determined that operation of the treatment unit comprising zeolite at lower temperature and higher LHSV (Liquid Hourly Space Velocity) should favor the desired alkylation reactions and inhibit the undesirable side reactions that generate benzene and toluene and indeed this phenomenon is what has been observed in operation. As a result, the undesirable co-production of benzene and toluene by the treater has been minimized by a unique start-up procedure employing relatively low temperature and/or relatively high LHSV, such a result not predictable from the prior art.